The US CDC published data late last week that raises hopes that this particularly severe influenza season may be peaking. Visits to doctors’ offices for flu-related illnesses are down, as are the number of reported positive flu tests. While it is too early to know whether influenza infections are truly decreasing or if this is a temporary lull, these preliminary data are encouraging. This week, we discuss what we have learned about the US flu season so far and offer perspectives on what these and other influenza-related data mean for future influenza preparedness efforts.

Influenza and Influenza-Like Illness Reports

Data collected via the CDC’s ILINet surveillance system found that, for the first time this flu season, the percentage of patients who sought care at a network of outpatient healthcare providers for influenza-like illness (ILI) decreased significantly from the previous week—from 7.5% to 6.4% of all patient visits. Similarly, clinical and public health laboratories reported lower percentages of positive influenza tests last week compared with the previous week. However, while the percentage of flu being diagnosed at laboratories seems to be falling, this is largely attributable to a decline in the number of specimens that are positive for influenza A. Conversely, the number of influenza B-positive specimens has been rising in recent weeks. We will need data over the coming weeks before it is possible to determine if the number of influenza-like illnesses and confirmed influenza infections are actually decreasing.

Pediatric Mortality

Reports of the number of children who have died as a result of influenza have been widely publicized in the media this flu season. To date, there have been 97 pediatric deaths reported in the United States—13 of which were reported last week. Death reporting generally lags even further behind other types of data, so it is too early to tell if the number of deaths reported this week are lower than in previous weeks. Similarly, the flu season is not yet over, so it is not yet possible to know how this season will compare to others. The CDC reports that since influenza-associated pediatric mortality was designated as a nationally notifiable condition in the US in 2004, the number of flu-related deaths in children each season has ranged from 37 to 171, with the exception of the 2009-10 H1N1 influenza pandemic, during which 358 pediatric deaths were reported in the US.

Vaccine Efficacy

Recently, the CDC reported that initial data indicated that the overall efficacy of the 2017-18 seasonal influenza vaccine is 36%, and 25% for influenza A(H3N2), which has been the dominant strain this season. Though these estimates show slightly more protection from the flu than health officials had initially expected, these numbers are clearly suboptimal. However, the interim estimates do provide evidence of other benefits of the flu vaccine that may be obscured by focusing only on its ability to prevent infection. Notably, the CDC has pointed out that the vaccine has been associated with a 59% reduction in H3N2-associated medical visits for influenza illness in vaccinated children.

These interim findings of the ancillary benefits of flu vaccines in protecting high-risk groups from serious illness or death track with findings from previous seasons. A review of the 2010-16 flu seasons published in the journal Pediatrics found that only half of flu-related pediatric deaths occurred in otherwise healthy children. In the majority of these cases (>80%), the children were not fully vaccinated. Similarly, research presented this week at the annual meeting of the American College of Cardiology found that individuals with heart failure who received the flu vaccine are 50% less likely to die from all causes during flu season than heart failure patients who do not get vaccinated.  This finding is based on a review of six studies conducted in the United States, Europe, and Asia, which collectively contained data on more than 78,000 patients.

Conclusion

While it is too early to declare that the current influenza season is on the decline, any evidence that the number of infections and healthcare visits may be decreasing is welcome news. This encouraging news may be offset, however, by the interim estimates of the effectiveness of the flu vaccine. While there are compelling data that the flu vaccine can play an important role in averting serious outcomes and death from the flu—particularly in high-risk groups—the low protection against infection for this year’s vaccine highlights again the importance of working to develop more effective vaccines. Fortunately, the NIAID published this week a plan for developing a universal flu vaccine, and a group of US Senators has proposed legislation that would invest $1 billion in research over the next 5 years. Hopefully, with better vaccines, the public health community will be better prepared for future seasonal influenza outbreaks.

 

Photo: Digitally colored negative-stained image of an influenza virion taken using a transmission electron microscope.

Photo courtesy of CDC/Frederick Murphy

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